It was really lucky that Harry now had Hermlone as a friend. He didn't
know how he'd have gotten through all his homework without her, what
with all the last-minute Quidditch practice Wood was making them do.
She had also lent him Quidditch Through the Ages, which turned out to
be a very interesting read. Harry learned that there were seven
hundred ways of committing a Quidditch foul and that all of them had
happened during a World Cup match in 1473; that Seekers were usually
the smallest and fastest players, and that most serious Quidditch
accidents seemed to happen to them; that although people rarely
died playing Quidditch, referees had been known to vanish and turn up months later in the Sahara Desert. (Harry Potter and
the Sorcerer's Stone)

In a sentence, the first clause has past tense, and the second past perfect. What semantic difference comes from the different tenses?

2 Answers
2

Reference Time is ambiguous—it may refer to the time of writing Quidditch Through the Ages or it may refer to the time at which Harry read the work—but that particular sort of ambiguity is inherent when discussing written works. For practical purposes, RT may be taken to equate the two times, as if Harry in reading is 'listening to' the book.

So all the simple pasts occur in Harry's 'present', RT; the past perfects may represent either simple pasts (if the sense is perfective) or present perfects (if the sense is stative) in that time.

So what Harry reads is something like this

There are seven hundred ways of committing a Quidditch foul. All of them happened during a World Cup match in 1473. (The past perfect represents a backshifted simple past, referring to a completed event.)

Seekers are usually the smallest and fastest players, and most serious Quidditch accidents seem to happen to them.

People rarely die playing Quidditch, but referees have been known to vanish and turn up months later in the Sahara Desert. (The past perfect represents a backshifted present perfect representing a present state of knowledge.)

X has been known [by somebody] to VERB is, as Michel Plungjan points out, an idiom with a stative sense, meaning approximately [somebody] possesses knowledge of past instances of X VERBing. So that last clause means

... but there are cases on record of referees vanishing and turning up months later in the Sahara Desert.

X is known to VERB, in the present tense, has a somewhat different meaning: [somebody] possesses knowledge that X VERBs.

I happened to meet an interesting sentence in The Scarlet Letter. ‘Pestilence was known to have been foreboded by a shower of crimson light.’ Is this more proper consulting your answer when it is said like this : ‘Pestilence had been known to be foreboded by a shower of crimson light.’
– ListeneverJun 21 '13 at 3:57

1

@Listenever Yes. McCawley remarks somewhere (I can't lay my eyes on it at the moment) apropos of complicated structures like this that have isn't so much a component of an ordinary "perfect" construction as it is a marker for "pastness". That seems to me to mean it can shuttle back and forth between the know piece and the to VERB piece—which is what you have just done!
– StoneyB on hiatusJun 21 '13 at 4:12

Most of this paragraph is written in the habitual past, meaning that the events described occur regularly. If Rowling had written:

referees vanished and turned up months later in the Sahara desert,

or

referees were known to vanish and turn up months later in the Sahara desert,

this would have implied that vanishing and turning up months later in the Sahara desert was something that happened fairly often to referees. I suspect the tense was changed to indicate that this was a relatively rare event.